X-ray photoelectron spectroscopy

Abstract

It is now some twenty-five years since the first commercial X-ray photoelectron spectrometers appeared in the market place and the intervening years have seen the technique develop into a mature, if not altogether routine, surface analytical method. However, the scientific roots of the method arc much older, the photoelectric effect was discovered by Hertz more than a century ago, and the energetics of the process described by Einstein some I8 years later! Although the development of XPS into a sophisticated analytical method is a result of the meticulous work of the Swedish Nobel Laureate Kai Siegbahn and his colleagues’, the first well documented reports of the production of photoelectron spectra are due to Robinson’ 4. who, using CuKa radiation, produced XPS spectra from several metals. Although the most prominent of the early workers, Robinson was not alone in the early days of XPS. The historical development of XPS from the work of Hertz in 1887. to the present day revitalisation of the technique by Siegbahn, has been discussed at length by Jcnkin et al’ ‘. These authors provide an excellent account of how XPS emerged over the period 1900l although not explicit in terms of analysis depth, they seemed to indicate the sampling depth of the XPS experiment to be less than IO nm. Thus the scene was set for the development of a technique which not only provided identification of the chemical state of a material but also gave a surface specific rather than a bulk analysis. Much of the pioneering work of Siegbahn’s group on the XPS analysis of solid organic and inorganic materials was published in a single volume in 1967 ‘. A companion volume on ESCA applied to free molecules was produced a year or so later’. Today X-ray photoelectron spectroscopy (XPS) is applied to a wide range of materials, in order to obtain a variety of analytical information. Although XPS can be applied to gases, liquids and solids, in this article we shall only be concerned with its relevance to solid surfaces and the contribution that the data produced can make in the fields of chemistry. physics and materials science. XPS is widely used in both surface science, that is the gleaning of knowledge concerning the composition and crystallography ol simple (often single crystal) surfaces and the modification of said surfaces in a well controlled manner, and surface analysis; the acquisition of analytical information concerning the outer surface layers of a.material. Information from the latter category is often related back to other process or materials parameters, in essence such an undertaking is applied science whilst the former discipline might be regarded as pure physics or chemistry. In this chapter WC shall consider the basic principles of X-ray induced photoemission, and the various levels of information that can be obtained from an XPS spectrum, and the manner in which such data should be interpreted. XPS is essentially a large area analysis (some mm ‘) with a characteristic analysis depth of several nanometers. Various experimental methods are available which can increase the spatial resolution to a few micrometres and the depth resolution to something approaching I pm, the ways in which such improvements in depth and spatial resolution may be achieved will be considered, together with recent dcvelopments in instrument design that have enabled significant improvements in chemical resolution to be gained in the early 1990s. Within the space constraints of the current chapter it is only possible to review briefly some of the more important aspects of the basic principles of XPS, before considering recent developments and applications. For a more detailed treatment of the physics of the process the reader is referred to the more advanced texts that have appeared ’ ” ’ ‘. Introductions to the method are also to be found in various monographs“‘.“. The utility of XPS in the characterisation of the surface and near surface regions of materials will be explored by way of a series of examples taken, in the main, from the author’s laboratory concerned with the application of XPS in areas 01 materials science such as adhesion. polymer chemistry, corrosion and composite materials.